Electric fence controllers are devices that control the delivery of electric voltage and current to wire fences. Electric fences are often used to prevent the passage of animals therethrough and are particularly useful for controlling grazing animals. Of major importance in fence controllers is the ability to deliver an electric charge high enough to repel or contain an animal, yet low enough to operate safely under all conditions.
Present electric fence controllers may be classified into three groups, continuous output, pulsed output, and load-sensing fence controllers. Continuous output fence controllers provide a sinusoidally varying voltage across their output terminals. Under certain safety standards, fence controllers are considered unsafe if they deliver an electrical output having too long an on-time, too short an off-time, or too high a root-mean-square (RMS) current per pulse. Continuous output fence controllers, by definition, have a zero off-time and an indefinite on-time and, therefore, do not satisfy such safety standards. An animal or human that touches a continuously electrified fence may not be able to let go or get away from the fence, raising the possibility of delivering an unsafe shock.
The second general class of fence controllers are pulsed output controllers. U.S. Pat. No. 4,114,185, issued Sep. 12, 1978 to Gallagher, discloses such a pulsed output controller. Such pulse controllers periodically deliver a high voltage pulse to the fence. Their periodic rate is often set to conform to off-time requirements of various safety standards. Thus, pulse controllers operate more safely than do continuous output controllers.
Unfortunately, pulsed output fence controllers do not always operate effectively. For instance, during the pulse controller's off-time, an animal can slip through the fence without receiving a shock.
In an attempt to overcome this disadvantage, a third general type of controller senses and discharges when the fence load increases considerably, such as when an animal contacts the fence. When the load remains in contact with the fence, though, this supposedly improved "load-sensing" controller operates as a pulse type controller, pulsing periodically and therefore, less effectively.
Besides possible ineffective operation, another disadvantage shared by pulsed output and load-sensing fence controllers is their generation of electromagnetic noise or interference. Both types of fence controllers deliver a high voltage pulse at their output. The high frequency portions of these pulses radiate from the fence as electromagnetic energy. The resulting electromagnetic energy can interfere with nearby radio communication and other electromagnetic transmission.
In addition to reduced safety and effectiveness and generation of electromagnetic interference, another disadvantage of existing electric fence controllers is their requirement of a ground terminal. To produce a shock, fence controllers require an animal to simultaneously touch (either directly or indirectly) a hot terminal and a ground terminal. The hot terminal or fence terminal is connected to a fence wire while the ground terminal is connected to a separate ground rod embedded in the earth. Even without a ground rod connection, however, existing fence controllers can deliver a shock to an animal contacting the hot fence wire by using the animal as the path to ground. Without an adequate ground, though, the animal may not receive a shock at all, or, if a shock is delivered, a high pulse current will be returned through the neutral of the AC power supply, likely damaging the fence controller and blowing its fuses. To operate at optimum safety, existing fence controllers therefore must be coupled to ground by installing a separate ground rod and connecting it to the fence controller's ground terminal.
Current fence controllers generally require the use of step-up transformers to produce a high-voltage output. Such transformers increase the weight, cost and labor needed to produce the fence controller.